1. Field of the Invention
The invention relates to denture adhesives and methods for making denture adhesives.
2. Description of Related Art
Dentures are substitutes for missing teeth and serve as replacement for all or some of the teeth found in the oral cavity. Despite diligent efforts by dental professionals and designers of dental prostheses, dentures do not always fit perfectly. Over time, even well-fitting dentures can become ill-fitting due to natural shrinkage and changes in the gum or mucous tissues. Therefore, adherent creams, liquids or powders are often used to secure or temporarily fix dentures within the mouth.
There are a number of desirable attributes of a denture adhesive composition. The denture adhesive should develop a high degree of tack upon contact with saliva so that the dentures can be held in place as soon as they are seated in the mouth. It is also highly desirable that the mucilage of the fixative be spread over the denture-mucosa interface in order to seal the denture in place effectively. The mucilage should possess sufficient cohesive strength to withstand the stresses of mastication, which act to rupture the seal and thus dislodge the denture. The denture adhesive must also exhibit sufficient resistance to degradation under the extreme environmental changes that can occur in the oral cavity during such common actions as drinking hot or cold beverages. Of course, the adhesive must also be releasable so that the denture wearer may remove the dentures for cleaning and maintenance. Denture adhesives are generally sold as a cream, liner or strip, liquid or powder, and many examples are well known in the art.
Early denture adhesives contained finely ground particles of natural gums that expanded when wet with water to become a viscous gel, which acted as a cushion and an adherent between the denture plate and the gum tissue. These denture adhesives, however, have been largely supplanted by polymeric denture adhesives in recent years.
U.S. Pat. No. 3,003,988 to Germann discloses a mixed partial salt of a methyl vinyl ether/maleic acid (or maleic anhydride) copolymer ("PVE/MA") as a denture adhesive. This mixed partial salt may be a calcium salt combined with a monovalent sodium, potassium, or quaternary ammonium salt with the calcium to the monovalent cation ratio from 2:1 to 10:1 on a weight basis (on a mole ratio basis the ratio was stated to be from 1:1 to 5:1), with the polymer being from about 50-95% neutralized by the cations.
The PVM/MA Ca/Na salts disclosed in examples I-V of Germann are prepared by a semi-dry process in the presence of a small amount of water and isopropanol which is insufficient to completely dissolve all of the PVM/MA anhydride and hydrolyze all of the anhydride groups to the acid form. Consequently, the equivalents of acid available to completely neutralize all of the metal hydroxides charged in making a high calcium containing Ca/Na salt by the semi-dry process in Germann are often insufficient. The result being that the PVM/MA Ca/Na salt is actually a mixture of unreacted metal hydroxides, PVM/MA Ca/Na salt, and unreacted PVM/MA anhydride which has a distinct taste associated with it.
There have been many alternatives and improvements to the original Germann polymer salt. U.S. Pat. No. 5,395,867 (Prosise--assigned to ISP) uses calcium, sodium, strontium, zinc, magnesium and potassium cations to change the properties of the polymer salts. Prosise discloses a "wet process" which employs a stoichiometric excess of acid groups (90-96% water), thus assuring complete dissolution of the PVM/MA copolymer and complete hydrolysis of the anhydride groups in the PVM/MA copolymer to acid groups. Consequently, the wet process yields a Ca/Na PVM/MA salt which has better organoleptic properties than a dry or semi-dry process salt. Additionally, dental adhesive compositions produced by the wet process also show to have improved elastic properties.
One important factor in designing a denture adhesive salt is consumer acceptance of the organoleptic qualities of the salts in the denture adhesive. Another factor is the ease of manufacture of the adhesive salts. A major disadvantage of the wet process in the prior art to produce organoleptically acceptable denture adhesives is the tendency of the process to produce precipitates of PVM/MA Ca/Na salt, which is particularly a problem in manufacturing PVM/MA salts with a high degree of substitution in calcium (about 69% and greater). While this precipitated PVM/MA salt appears to have no activity as a denture adhesive salt, the salt precipitates when formed in large enough quantities in the salt-making reactors can plug reactor lines and pumps used to transfer the product to dryers causing expensive maintenance and down-time. Therefore, it is not possible to make dental adhesive compositions with a high degree of substitution because of plugging problems which basically stop the manufacturing process.
Prosise teaches that one of the copolymers from the copolymeric Ca/Na mixed salt are derived from is available as GANTREZ.RTM. AN, supplied by International Specialty Products (ISP, the assignee of the Prosise patent). Product literature from ISP/GAF Corporation (page 11, Gantrez.RTM. AN, technical bulletin 7543-017) discloses that the addition of calcium beyond 0.7 mol equivalents causes precipitation of Gantrez.RTM. AN. The problem can be avoided by not using more than 0.7 mol equivalents of calcium, i.e., avoiding the precipitation formed during further calcium neutralization "by replacement of calcium with caustic."
The inventor has surprisingly found that denture adhesive compositions can be manufactured comprising calcium/sodium PVM/MA salts formed via a wet process with the level of calcium beyond 0.7 mol equivalents. It is found that it is NOT necessary to avoid the precipitation, i.e., manufacturing problems by avoiding the addition of calcium beyond 0.7 mol equivalents. Contrary to the prior art teaching of "replacement of calcium with caustic," the addition of calcium well beyond the 0.7 mol equivalents (70% degree of substitution) is possible by controlling the amount of sodium and/or potassium hydroxide employed in producing calcium/sodium and/or potassium PVM/MA salts in the wet process.
There is also provided in the invention a denture adhesive paste comprising a mixed partial calcium/sodium or calcium/potassium salt of a PVM/MA wherein the degree of substitution of the copolymer in calcium is at least about 69%.